1. Field of the Invention
The subject invention relates to methods and apparatus for filling a cavity with a liquid through a hypodermic needle and also to pressure transducing and pressure transducers, and domes for pressure transducers.
2. Prior-Art Statement
The prior art, like the subject invention, are herein primarily described in terms of physiological pressure transducers, although the scope of the subject invention obviously is not limited to such devices.
As apparent from U.S. Pat. No. 4,063,553 domes for physiological pressure transducers are typically provided with two ports; the second port facilitating filling of the internal dome cavity with saline solution, or other liquid, and the removal of all air bubbles after the dome has been installed on the transducer. For miniature transducers, it is generally desirable to use domes with a single, preferably coaxial port.
Since filling such domes is difficult after installation, they are frequently filled from the open end prior to attachment to the transducer. Typically, a syringe with a hypodermic needle is used for this purpose.
Such a procedure is awkward and untidy for reusable domes and impossible for domes with isolation membranes which could be pierced by the hypodermic needle.
Several patents located in the course of a novelty search have been considered, and found devoid of a solution of the above mentioned problem.
For instance, U.S. Pat. No. 3,565,056 provides a permanently attached conduit for supplying a saline solution to the chamber and internal parts of a strain-gage sensor assembly and catheter during pressure measurements. U.S. Pat. No. 3,631,850 proposes the use of a valve for filling the interior chamber and the bores of a transducer with a pressure-transmitting liquid. U.S. Pat. No. 3,811,429 proposes the use of a hypodermic needle or filling a cavity at a pressure transducer with a liquid. In particular, that patent proposes an internally threaded port into which a closing screw may be inserted, and a channel extending between that port and the transducer cavity at an angle to the internally threaded port. For the placement of a liquid into the transducer cavity, it is proposed to remove the screw and to insert a hypodermic needle or the like into contact with the obliquely extending channel defining the filling port. In practice, air or other gases may be entrapped in or at the cavity in such a structure and difficulties are encountered in actually transferring the injected liquid from the threaded port to the obliquely extending channel and hence to the transducer cavity.
U.S. Pat. No. 3,865,100 proposes the use of two ports, one for attachment to an injection syringe or catheter and the other, extending laterally into the transducer, for the supply of a physiological saline solution to the transducer cavity.
Reference may also be had to U.S. Pat. Nos. 3,157,201, 3,623,479, 3,731,680, 3,807,142, and 3,996,027, which have been cited in connection with the above mentioned U.S. Pat. No. 4,063,553, but are not considered particular pertinent in the present context.